Studies have been conducted regarding secondary batteries used in portable electronic devices and hybrid vehicles. As typical secondary batteries, lead storage batteries, alkali storage batteries, lithium ion batteries, and the like are known. Among a variety of secondary batteries, lithium secondary batteries in which a lithium ion battery is used have advantages of high output, high energy density, and the like.
As a cathode material used for lithium ion batteries, phosphate which includes Li and transition metals and has an olivine structure is known. For example, iron-based olivine-type compounds have excellent electrochemical characteristics and stability, and thus, currently, are employed for a variety of uses such as cathode materials for stationary or in-vehicle lithium ion batteries.
In recent years, there has been a demand for improving the performance of lithium ion batteries, and a variety of studies have been performed. For example, in a case in which a lithium ion battery is used in a high-current density region, additional improvement of electron conductivity is required for improving performance. In response to the requirements of the above-described properties, techniques of coating the surfaces of cathode active materials with a carbonaceous material (hereinafter, in some cases, simply referred to as carbon coating) are known (for example, refer to Patent Document 1 to 3). As a method for coating the surfaces of cathode materials with carbon, a method is known in which a carbon source and a cathode material are mixed together and the mixture is fired in an inert atmosphere or a reducing atmosphere.